Various types of sensors have been known and one such sensor is described, for example, in U.S. Pat. No. 4,157,282, Riddel. This sensor has a plate-like solid electrolyte body, on the opposite major sides of which electrodes are applied which are exposed to the gas to be tested or analyzed. A voltage signal is provided by the electrodes which depends on the oxygen content of the gas. This type of sensor is referred to as a potentiometric sensor since the sensor element itself provides an output voltage in dependence on the oxygen content of the gases. U.S. Pat. No. 3,691,023, Ruka et al., describes a sensor which likewise uses a plate-like solid electrolyte body on which electrodes are applied to the major surfaces, and located transverse to the stream of gas to be tested. A voltage is applied to the electrodes of this sensor. The current flowing to the sensor will be dependent on the oxygen of the gas. The current will be limited by gaseous diffusion. This sensor is referred to as a polarographic sensor. It is supplied with a heating element to set a certain and predetermined working temperature in order to heat the solid electrolyte body to its operating temperature. Another type of sensor is described in U.S. application Ser. No. 006,093, filed Jan. 24, 1979, now abandoned, Dietz, based on German Disclosure Document DE OS No. 27 11 880, in which the sensing electrode is covered with a porous coating to form a diffusion barrier for the oxygen molecules of a test gas. This sensor is of tubular shape, closed off at one end, and utilizes a reference gas, for example the oxygen of ambient air, which is supplied to the interior of the closed tube.